The Exorcism is Russell Crowe’s second starring vehicle featuring him in a priest’s outfit and becoming possessed by a demon at the end of the film. Unlike Crowe in The Pope’s Exorcist, though, in The Exorcism he plays an actor named Anthony “Tony” Miller, who takes on the role of a priest for an exorcism horror movie. As shooting begins for the project, Tony and his daughter, Lee (Ryan Simpkins) who have a complex relationship after Tony became an alcoholic and addict as his wife was dying of cancer begin to experience telltale signs of a demonic presence.

https://screenrant.com/the-exorcism-ending-explained/